Mutations in Drosophila have identified genes controlling major steps in development. The function of one of these genes, engrailed, is required to establish and maintain developmental compartments within each segment of the embryo and the adult. Molecular analysis has revealed that this gene is expressed in an exceedingly intricate spatial and temporal program. This program of expression appears to be the result of regulation in trans by a number of other developmental genes and in cis by about 65 kb of flanking sequences. It has been argued that a conserved protein domain that is found in the products of several Drosophila developmental genes might include a DNA binding domain. We found that a fusion protein that included the engrailed homeo domain behaved as a sequence specific DNA binding protein. We propose that the protein products of developmental genes having homeo domains are DNA binding proteins and regulators of transcription. Further, because these regulators function combinatorially we think that they interact. We propose experiments to test for the interaction of these regulators with DNA and with each other. We will directly measure physical interaction in vitro and design tests for functional interaction in vivo. We think that these interactions will prove fundamental because the elaborate process of pattern formation is presumably founded upon the regulatory interactions among the products of the developmental genes.